Matilija Sandstone

The type locality of the unit is at Matilija Hot Springs, along the Ventura River about northwest of Ojai, near to California State Route 33. The sandstone layers are made up of well-sorted grains of quartz and feldspar. It accounts for the highest peaks in the Santa Ynez range, dipping underneath the younger Coldwater Sandstone, Cozy Dell Shale, and Sespe Formation near San Marcos Pass in the center of the range. The thickness of the unit is widely variable, generally decreasing to the west. In its type locality it is around thick; in the vicinity of La Cumbre Peak it is thick; and while it thins to only thick underneath San Marcos Pass, it thickens again to around at the high summits of Santa Ynez and Broadcast Peaks. It thins westward from there, being only thick at Refugio Pass, and less than 300 in the Santa Rosa Hills and west to Point Conception. Along with the younger Coldwater Formation, the Matilija is the source of the enormous sandstone boulders which are found abundantly along the creeks and shoreline of Santa Barbara and Goleta. Additionally, boulders are strewn on hillsides in the upper Riviera, and elsewhere on hillsides and floodplains. These boulders, none of which can be moved even in the most intense modern-day flood events, tumbled down the mountains during Pleistocene-age storms and mudflows of unimaginable intensity. Boulders of the Matilija were included in the mud and debris flows in the January 2018 Montecito mudflows. The Matilija Sandstone produces little soil when it weathers, and generally supports only hard chaparral on slopes that are not bare rock. North-facing slopes have some stands of pine and fir in the higher elevations. In the Molino Offshore Gas Field, southeast of Gaviota about offshore, the formation is at an average of below the seafloor, and a well drilled to had not reached the bottom of the formation. ==Deposition environment and tectonic history==

Sometime in the early Eocene, around 50 million years ago, the landmass containing present-day Santa Barbara County became submerged, allowing sediment deposition offshore. During this time the sea alternately deepened and became shallow again, with the deposition environment supporting either sandstones or shales corresponding to shallow or deep conditions. The Matilija Sandstone dates from the earliest sustained shallow-water episode during this time, the period from 48.5 to 46 million years ago. After reaching its current position along the coast, the entire Santa Ynez Range was uplifted, mostly during the Quaternary period and late Pliocene, by crustal shortening and compression from tectonic forces at the boundary of the Pacific and North American Plates. The Matilija Sandstone and other units, formerly submerged, rose over from their original positions. In some places the forces causing this uplift were so abrupt that sedimentary layers deposited as recently as the Pleistocene are now angled 60 degrees from horizontal. ==Paleontology==

While the formation is rich in microfossils, it is considered to be of low paleontologic sensitivity, i.e. larger fossils are unlikely to be encountered. In the Santa Rosa Hills, the unit has yielded numerous shells of molluscs, including bivalves and gastropods. ==Economic importance==

As a potential reservoir for oil and gas deposits, the Matilija Sandstone is grouped with the above-lying Cozy Dell Shale and Coldwater Sandstone as the Gaviota-Sacate-Matilija Sandstone Play. The likely source rocks for any petroleum accumulation in the Gaviota-Sacate-Matilija Play are the lower-lying organic-rich shale units, such as the Juncal-Anita Formations. Since this deep-lying play is incompletely explored, with many boreholes not even reaching the Matilija, the estimates for hydrocarbon resources are speculative, with the United States Bureau of Ocean Energy Management, Regulation and Enforcement giving a range of of oil recoverable using current technology for the entire Santa Barbara-Ventura Basin Province. In general, in the eastern and central parts of the basin, the sandstone would be too deeply buried to have reservoir potential. However, the area potentially containing oil and gas is large, including approximately within the Federal Outer Continental Shelf. The largest single hydrocarbon accumulation yet identified in the Matilija Sandstone is in the Molino Offshore Gas Field, discovered in 1983, and contains approximately of oil equivalent. ==See also==